This invention relates to a method of electrically cutting a work piece with a wire electrode in which a work piece is electrically cut with a wire electrode in a manner similar to that of cutting with a fret saw, while an electric current is fed between the wire electrode and the work piece through a cutting liquid in a cutting gap. The invention provides a novel method especially effective in improving the aforementioned method and an apparatus for practicing the same.
FIG. 1 is an explanatory diagram for a description of the operation of a conventional apparatus for practicing a method of taper-cutting a work piece by electrical discharge.
The apparatus, as shown in FIG. 1, comprises, a table 3 on which a work piece 1 to be cut is placed; the table 3 being moved in X and Y directions respectively by an X-axis drive motor 4 and a Y-axis drive motor 5. A wire electrode 2 is supplied from a wire supplying reel 7, and then wound on a wire winding reel 12 passing through a tension roller 8, a current supplying section 9, an upper wire guide 10 and a lower wire guide 11 in the stated order.
The apparatus of FIG. 1 further comprises an x-axis drive motor 14 and a y-axis drive motor 15 adapted to drive the wire guide 10 respectively in x and y directions in order to incline the wire electrode 2 at a desired angle in a desired direction.
A cutting liquid is supplied from a cutting liquid supplying device (not shown) into a cutting gap formed between the wire electrode 2 and the work piece 1 during the cutting operation. A cutting electric power is supplied from a power source 16. A discharge circuit, which is made up of a DC power source 17, a charge current limiting resistor 18 and a capacitor 19 is usually employed as the power source 16.
A control device 20 is provided to control the X-axis drive motor 4 and the Y-axis drive motor 5 adapted to drive the table 3 in the X-Y direction, and the x-axis drive motor 14 and the y-axis drive motor 15 adapted to drive the upper wire guide 10 in the x-y direction. These motors are used to move the work piece 1 relative to the wire electrode 2 so that the work piece 1 is cut into a desired configuration. The control device 20 is made up of a profiling control device, an N/C control device or a computer.
FIG. 2 shows a work piece being subjected to taper-cutting by the apparatus of FIG. 1 to produce a die. In FIG. 2, reference character 1A designates the die which is obtained by cutting the work piece 1. The periphery of the lower opening of the die 1A forms the cutting edge. The area of the upper opening of the die is larger, as defined by a value r, than the lower opening, so that the inner surface has a relief taper in the finished condition.
If the thickness of the work piece 1 is represented by t, then the taper angle, i.e. the inclination angle .theta. of the wire electrode 2 is: EQU .theta.=tan.sup.-1 (r/t) (1)
Accordingly, in forming the die 1A, it is necessary at all times to incline the wire electrode 2 outwardly by the angle .theta. in a plane perpendicular to the cutting surface of the work piece. In other words, it is necessary to drive the upper wire guide 10 in the x-y direction by controlling the x-axis drive motor 14 and the y-axis drive motor 15 so that the wire electrode 2 is maintained inclined by the angle .theta. in a direction perpendicular to the cutting line.
The wire electrode 2 must be inclined as stated above at all times. When the cutting operation is carried out along the straight line of the die 1A, it is unnecessary to change the inclination direction of the wire electrode 2. However, when the cutting operation is carried out along corners or curves, it is necessary to change the inclination direction of the wire electrode in such a manner that the inclination direction coincides with a direction perpendicular to the cutting lines. This is accomplished by driving the wire guide 10 in the x-y direction according to the advancement of the cutting operation.
That is, in the case of FIG. 2, the inclination direction of the wire electrode 2 must be changed according to the advancement of the cutting operation. While the cutting locus is advanced from the point b to the point c on the lower surface of the work piece 1, it must advance from the point B to the point C on the upper surface of the work piece 1.
FIG. 3 is an enlarged view showing the movement of the wire electrode 2 with respect to the work piece 1, which is effected when the cutting operation is carried out along the curve. The work piece 1 is cut by moving the wire electrode 2 along the surface of a circular cone with the vertex a.
If, in this case, the curvature radius of the locus of the wire electrode 2 which moves from the point b to the point c on the lower surface of the work piece 1 is represented by Rd, then the curvature radius Ru of the locus of the wire electrode 2 which is moved from the point B to the point C on the upper surface of the work piece 1 is: EQU Ru=Rd+r=Rd+t.multidot.tan .theta. (2)
Accordingly, the cutting distance per unit time of the upper surface of the work piece 1 is different from that of the lower surface: that is, the cutting speed of the upper surface is different from that of the lower surface.
In electric discharge machining (EDM) with the aid of the wire electrode, the amount of material cut away from the work piece by the application of current is a function of the cutting time. Therefore, if the cutting speed is different, the width of the cut groove becomes different.
FIG. 4 is a plan view showing the cutting operation along the curve. As is apparent from FIG. 4, on the upper surface of the work piece 1, curve cutting is carried out at a speed equal to substantially the maximum speed which is employed for linear cutting. Accordingly, the width of the groove cut along the curve from the point B to the point C is equal to the width of the groove cut along the straight line up to the point B, as shown by solid line in FIG. 4.
On the other hand, on the lower surface of the work piece 1, the curve cutting speed is lower than the straight line cutting speed and the width of the cut groove is increased. Accordingly, the width of the groove cut along the curve from the point b to the point c is larger than the width of the groove cut along the straight line up to the point b, as shown by dotted line in FIG. 4. That is, the dimensional accuracy of the curve on the lower surface of the work piece becomes lower than the accuracy for the upper curve.
As described above, in forming a die 1A, the cutting edge thereof which is formed by the lower surface of the work piece 1 and must be of highest accuracy, becomes in fact low in dimensional accuracy. This is a fatal drawback and makes it difficult to practice the method of taper-cutting a work piece by electric discharge.